This invention pertains to X-ray image subtraction method and apparatus for performing such methods, more commonly called digital subtraction angiography (DSA).
Digital subtraction angiography is an X-ray procedure for visualizing blood vessels in the body. The procedure involves making an X-ray image, called a mask image, of an anatomical region containing blood vessels of interest. The mask image is digitized and the digital data representative of the picture elements (pixels) in the mask image are placed in a digital frame memory. At some time, usually just before the mask image is obtained, an X-ray contrast medium, such as an iodinated compound, is injected into the blood circulation system. When the contrast medium reaches the blood vessels in the region of interest, a series of X-ray images are made and they are converted to digital data. The mask or precontrast image data are then subtracted from the postcontrast image data to cancel or subtract out all soft tissue and bone structure common to both images in which case data representative of the contrast medium filled blood vessel remains. The method just outlined is commonly called temporal subtraction imaging because of the substantial time lapse between acquisition of the pre-contrast image or a series of them and the postcontrast image or a series of them.
One of the problems associated with temporal subtraction techniques is that there may be a substantial loss of registration between the pre-contrast mask and post-contrast images due, primarily, to movement of soft tissue. Movement of soft tissue or anything else during acquisition of the pre-contrast and post-contrast images will result in artifacts appearing in the display of the subtracted or difference image data and these artifacts obliterate the desired image of the blood vessel whose interior walls are defined by the contrast medium.
With temporal subtraction it is often possible to achieve good cancellation or subtraction of bone, which usually does not move involuntarily, but some artifacts or misregistrations of pixels between the two images may result from involuntary tissue motion such as that due to swallowing, breathing, peristalsis and blood vessel expansion and contraction.
Another image subtraction technique is commonly called energy subtraction. Energy subtraction is based on the fact that X-ray attenuation by a body or any material is an X-ray energy dependent phenomenon and that the energy dependence is different for materials having different atomic number averages. In energy subtraction, an X-ray image of an anatomical region containing a blood vessel is obtained with a nominally low kilovoltage (kV) applied to the X-ray tube so the beam projected through the body has an energy spectral distribution within a band having low average energy. After the relatively low energy image is obtained, another image is obtained with a comparatively higher kV applied to the X-ray tube so the spectral band has a higher average energy. The two images in a pair can also be made in the opposite order, that is, the high energy exposure can precede the low energy exposure. For the sake of consistency and convenience, it will be assumed herein that the low energy exposure is the first one in a pair. The low and high energy images are acquired in a very short time interval, typically within 33 milliseconds, compared to a comparatively long time interval, typically 10 seconds between pre-contrast and post-contrast in temporal subtraction. Thus, the possibility of artifacts caused by patient motion is greatly reduced with energy subtraction. For ordinary tissue studies the two images may be made in the absence of any contrast medium. For angiographic studies, the two images are obtained when there is an X-ray contrast medium such as an iodinated compound present in the blood vessels. In any case, the high average energy image pixel data are subtracted from the low average image data and data representative of the difference between the two images remains. Prior to subtraction, the data are variously weighted or scaled to bring about cancellation of soft tissue by subtraction. The data could be weighted to bring about cancellation of bone also. However, it is not possible to cancel boney structures without also subtracting out most of the iodinated contrast medium which is really what one is depending on to define and allow visualization of the blood vessel.
A method called hybrid subtraction uses a combination of energy and temporal subtraction techniques. In hybrid subtraction, X-ray images are obtained at two different average X-ray energies, that is, with two different kilovoltages applied to the X-ray tube and the images are combined in a manner to suppress signals due to soft tissue. Methods for obtaining X-ray beams having low and high average energies for energy spectral bands are well known. One way is to apply a constant kilovoltage (kV) to the X-ray tube anode and interpose two different filters alternatingly in the beam. One filter is for softening the X-ray beam, that is, for removing high energy spectra above a low average energy band. One filter should have relatively low attenuation for X-ray photons having energies below the k-edge and high attenuation for energies above the k-edge to thereby remove high energy spectra. The other filter hardens the high energy beam and is composed of a material that attenuates or absorbs the low energy spectra intensely.
Low and high average energy X-ray beams can also be obtained by applying low kV and alternately, high kV to the X-ray tube anode. A preferred way for generating low and high energy X-ray beams is to switch the X-ray tube applied voltage and filters correspondingly.
In hybrid subtraction, a mask is obtained first by projecting a low average energy X-ray beam (hereafter called low energy beam or low energy spectral band) through the body followed by a higher average energy X-ray beam (hereafter called high energy beam or high energy spectral band) when X-ray contrast medium injected into the blood circulation system has not yet entered the blood vessels in the anatomical region being examined. The signals corresponding to the pixel intensities composing the image which consist primarily of bone and soft tissue acquired at the two energies, are scaled or weighted using appropriate constants (k), and then the images are subtracted to produce a mask image in which signals due to soft tissue variations are suppressed and boney structures remain. The data for a pair of high and low energy X-ray images are next obtained when the injected iodinated compound constituting the contrast medium reaches the vessels in the examination region. The data for this pair of images are acted upon by the same constant weighting factors that were used with the first pair of images and one image in this pair is subtracted from the other such that the resulting post-contrast image contains data representative of bone plus vessels containing contrast medium. The final step in hybrid subtraction is equivalent to temporal subtraction and involves subtracting the dual energy post-contrast image from the dual energy pre-contrast mask image to thereby suppress or cancel the bone and isolate the contrast medium containing vessels. A major advantage of hybrid subtraction over temporal subtraction alone is the reduced sensitivity to soft tissue motion artifacts because the soft tissue is suppressed or cancelled in both dual energy images.
Hybrid subtraction is a good technique for eliminating anything that may have moved during the time between obtaining the mask image and the postcontrast image or images. However, if there is no movement during ordinary temporal subtraction, wherein the post-contrast low or high energy images are simply subtracted from the pre-contrast mask image, then temporal subtraction images can be used because they generally have a better signal-to-noise ratio (SNR) than hybrid subtraction images. A higher SNR results in displayed images that have better contrast at a given noise level.
It is known that in some examination procedures there is a high probability that there will be soft tissue motion. Examination of the contralateral bifurcation of the carotid artery is one example. As many as fifty percent of the standard DSA carotid examinations result in images having soft tissue motion artifacts due to patient swallowing or an involuntary reaction to the presence of iodinated contrast medium. While hybrid subtraction is successful in eliminating these artifacts, the resulting image has an inherently lower SNR than temporal subtraction. Attempts have been made to improve SNR such as by image integration and noise reduction techniques, with varying degrees of success. Since noise in the hybrid subtracted image makes diagnosis for X-ray image interpretation more difficult, heretofore it has been the practice to provide for displaying a temporally subtracted image and the hybrid subtracted image separately and simultaneously. A temporal subtracted image provides the best overall image quality except where motion artifacts are present. A hybrid subtracted image has the motion artifacts eliminated but this image has more noise or a different noise texture than the temporal subtracted image. By providing both hybrid and temporal images on separate films or on separate video monitor screens, the interpreting radiologist could refer to both images and make a judgment as to the condition of a blood vessel that might be obscured wholly by motion artifacts in the temporal subtraction image and diminished by noise or low SNR in the hybrid image.
A second problem with hybrid DSA results from having the technician make a judgment as to a proper weighting coefficient or constant, k, to bring about elimination of motion artifacts when the images are subtracted. This is done empirically while viewing the result on a video monitor and often adds significantly to the total examination time. Moreover, an unskilled technician could fail to select the optimal weighting factors and thus not realize the full benefit of the hybrid subtraction technique.